Unverified Commit 05d450aa authored by Song Shuai's avatar Song Shuai Committed by Palmer Dabbelt
Browse files

riscv: Support RANDOMIZE_KSTACK_OFFSET 



Inspired from arm64's implement -- commit 70918779
("arm64: entry: Enable random_kstack_offset support")

Add support of kernel stack offset randomization while handling syscall,
the offset is defaultly limited by KSTACK_OFFSET_MAX() (i.e. 10 bits).

In order to avoid trigger stack canaries (due to __builtin_alloca) and
slowing down the entry path, use __no_stack_protector attribute to
disable stack protector for do_trap_ecall_u() at the function level.

Acked-by: default avatarPalmer Dabbelt <palmer@rivosinc.com>
Reviewed-by: default avatarKees Cook <keescook@chromium.org>
Signed-off-by: default avatarSong Shuai <songshuaishuai@tinylab.org>
Link: https://lore.kernel.org/r/20231109133751.212079-1-songshuaishuai@tinylab.org


Signed-off-by: default avatarPalmer Dabbelt <palmer@rivosinc.com>
parent dded618c

arch/riscv/Kconfig

+1 −0
Original line number Diff line number Diff line
@@ -109,6 +109,7 @@ config RISCV 
	select HAVE_ARCH_KGDB_QXFER_PKT

	select HAVE_ARCH_MMAP_RND_BITS if MMU

	select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT

	select HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET

	select HAVE_ARCH_SECCOMP_FILTER

	select HAVE_ARCH_THREAD_STRUCT_WHITELIST

	select HAVE_ARCH_TRACEHOOK

arch/riscv/kernel/traps.c

+16 −1
Original line number Diff line number Diff line
@@ -6,6 +6,7 @@ 
#include <linux/cpu.h>

#include <linux/kernel.h>

#include <linux/init.h>

#include <linux/randomize_kstack.h>

#include <linux/sched.h>

#include <linux/sched/debug.h>

#include <linux/sched/signal.h>
@@ -310,7 +311,8 @@ asmlinkage __visible __trap_section void do_trap_break(struct pt_regs *regs) 
	}

}



asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)

asmlinkage __visible __trap_section  __no_stack_protector

void do_trap_ecall_u(struct pt_regs *regs)

{

	if (user_mode(regs)) {

		long syscall = regs->a7;
@@ -322,10 +324,23 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs) 


		syscall = syscall_enter_from_user_mode(regs, syscall);



		add_random_kstack_offset();



		if (syscall >= 0 && syscall < NR_syscalls)

			syscall_handler(regs, syscall);

		else if (syscall != -1)

			regs->a0 = -ENOSYS;

		/*

		 * Ultimately, this value will get limited by KSTACK_OFFSET_MAX(),

		 * so the maximum stack offset is 1k bytes (10 bits).

		 *

		 * The actual entropy will be further reduced by the compiler when

		 * applying stack alignment constraints: 16-byte (i.e. 4-bit) aligned

		 * for RV32I or RV64I.

		 *

		 * The resulting 6 bits of entropy is seen in SP[9:4].

		 */

		choose_random_kstack_offset(get_random_u16());



		syscall_exit_to_user_mode(regs);

	} else {